Accurate and precise measurement of optical spectra is important to numerous fields. This includes measurements of the dynamical properties of hydrodynamic systems by light scattering, examination of the convoluted spectra characteristics of thin fluid films under shear or subject to temperature gradients, laser doppler anemometry, and fiber optic communications. More generally, such measurements are useful in systems which modulate or demodulate the phase, amplitude or frequency of an optical signal, or which phase lock two or more optical oscillators.
Quite often, the spectral distribution of the optical signal is not symmetric about a center frequency. Available systems, however, seem not to be well suited for measuring such asymmetric spectra.
One such system has been described by Bernard S. Glance in his article "An Optical Heterodyne Mixer Providing Image-Frequency Rejection," published in the Journal of Lightwave Technology, Volume LT-4, No. 11, November of 1986 at pp. 1722-1725. Glance proposes to use a beam splitter to add the signal to a local oscillator signal. The merged beam enters a polarizing beam splitter which separates the two orthogonal polarization components of the beam. Each of the two exiting beams is then detected separately by a photodiode, and the two resulting currents are added.
The Glance system, unfortunately, requires a very strong local oscillator in order to work well, typically 10 to 100 times the magnitude of the test signal. This seriously limits the available dynamic range and injects substantial noise.
Another system is disclosed by N. Walker and J. Carroll in their article "Simultaneous Phase and Amplitude Measurements on Optical Signals Using A Multiport Junction," published in Electronics Letters on Nov. 8, 1984, Volume 20, No. 23 at pp. 981-83. Four beam splitters are arranged to form an eight-port junction. The measurement of the power output from the eight ports can then be used to calculate the phase and amplitude of the signal relative to the reference.
In this system, however, the spacing between the beam splitter cubes is critical. Since these cubes are traditionally affixed to a common substrate which expands and contracts with changes in temperature, the system has a significant degree of temperature instability and associated drift.